If Microsoft was waiting for the other shoe to drop in its patent lawsuit against TomTom, it did -- but it may not have been the shoe Microsoft expected.

Microsoft in late February sued TomTom for what it claims are violations of eight separate patents, after what Microsoft officials claims was a year of attempts to negotiate with the GPS vendor.

But today, Netherlands-based TomTom countersued Microsoft (NASDAQ: MSFT) in the U.S. District Court for the Eastern District of Virginia.

TomTom's countersuit alleges Microsoft is infringing four of its own patents, although it's not possible from the patent titles to tell whether they are directly related to any of the eight Microsoft patents that the software giant alleges TomTom infringed.

TomTom's countersuit concerns the use of its patents in Microsoft Streets and Trips, and carry titles like "Generating a Maneuver at the Intersection Through a Turn Lane."

The spat between Microsoft and TomTom drew the attention of some in the open source industry. Three of the patents that Microsoft's suit claims TomTom infringes involve Microsoft's FAT (File Allocation Table) file system, and that's where Linux comes in.

Many open source advocates and other company critics viewed Microsoft's lawsuit as a sneak attack in a much larger and darker war with the open source operating system. Microsoft officials have discounted that, while many observers point out that Microsoft has sued only a handful of companies in total for patent infringement through the years. Instead, the company prefers to work out patent licensing or cross-licensing deals with alleged offenders.

Yet some observers argued that TomTom is not an obvious Linux company to attack, although its navigation devices run a version of the OS. Red Hat would make a much larger target, for instance, some analysts have pointed out.

Observers also said that Microsoft has gradually been shifting some of its own development to an open source approach, including its own CodePlex site and partnerships with other sites, including SourceForge.

However, open source advocates have countered that those are token responses to a rapidly-expanding movement.

To them, that makes TomTom's countersuit -- even though none of the four patents named in it appear to have anything to do with Linux -- an important play in a much larger game.

Microsoft's critics like to point out that CEO Steve Ballmer and other senior company executives, at least until recently, continually trotted out anti-Linux and anti-open source diatribes for years. Ballmer accused Linux vendors of violating as many as 235 Microsoft patents but would never delineate what patents were infringed.

This may be open source advocates' chance to bring some of those patents out into the open.

"TomTom has decided to fight -- and perhaps fight hard," Andrew Updegrove, a partner in Gesmer Updegrove and a board member of the Linux Foundation, said in an e-mail to InternetNews.com. He sees Microsoft's dual behaviors as signs that the company is beginning to come apart over its own internal indecision regarding open source.

"Just as the iron curtain eventually fell when the illusion of the Soviet economic model could no longer be sustained, I believe that Microsoft's anti-[Free and Open Source Software] strategy will eventually collapse as well -- not from assault from the field, but due to an uprising from within," Updegrove added.

"We are reviewing TomTom's filing, which we have just received. As has been the case for more than a year, we remain committed to a licensing solution, although we will continue to press ahead with the complaints we initiated in the U.S. District Court for the Western District of Washington and the International Trade Commission," Horacio Gutierrez, Microsoft deputy general counsel of intellectual property and licensing, said in the posting.

Despite critics' nagging suspicions about Microsoft's motives, though, some analysts don't see the two suits as having much to do with ideology. Instead, it appears to be two companies trying to be responsible to their shareholders.

"It's not as much about open source or Linux as much as it is about protecting true intellectual property," Tim Bajarin, president of Creative Strategies, told InternetNews.com.

Charles King, principal analyst at Pund-IT, agrees.

"I'm not seeing any indication from Microsoft that TomTom is a way for them to make more aggressive attacks on Linux," King told InternetNews.com. "A large percentage of the Fortune 1000 companies are now running significant numbers of Linux servers in their data centers, so I think that anything having to do with open source patents would be counterproductive at this point."

Bajarin also said that TomTom's countersuit may actually a negotiating technique.

"A lot of this seems like it might have an undercurrent of trying to reach some settlement," Bajarin added.